1. Field
The present application relates generally to the operation and design of analog front ends, and more particularly, to the operation and design of direct conversion transmitters that utilize passive mixers.
2. Background
Direct conversion transmitters typically use active mixers to up-convert baseband signals to radio frequencies (RF). Active mixers provide high conversion gains and excellent isolation between the mixer's input and output ports. However, active mixers typically require large power supplies and may have problems with noise performance.
Passive mixers address some of the problems exhibited by active mixers. For example, passive mixers can operate with smaller power supplies and exhibit better noise performance than active mixers. However, passive mixers typically do not provide the same reverse isolation as active mixers. Thus, passive mixers are generally not used in direct conversion transmitters due to the interaction of the complex impedances on the mixer's baseband and RF ports, which may affect the amplitude flatness symmetry across the transmission band.
Therefore, it would be desirable to have a way to utilize a passive mixer in a direct conversion transmitter that overcomes the problems associated with reverse isolation described above.